On 10/26/2022 David R Laws in Australia wrote:
"I recently purchased a new water pump for the car and was rewarded with a significant over heating problem. It turned out the impeller had not been pressed down to give the 20 thou clearance between the impeller and the body of the pump – the gap was in fact 65 thou !  The impeller was doing virtually nothing to circulate water through the engine. Anyone fitting a new pump would be well advised to check that critical clearance". -David

Illustration above from MG Series MGA Workshop Manual (for pushrod engines).
Illustration below from MG Series MGA Twin Cam Workshop Manual.

I recon 0.030-inch may be a bit excessive. I'd be inclined to keep it under 0.020-inch.

On 10/29/2022 David R Laws wrote:
"The Workshop manual does have a technical drawing which notes the clearance of the impeller head from the pump body at between 20-30 thou. At the end of the day you still have to know there might be a specific problem before you can pull out the feeler gauges and go looking for it. I would not be alone in assuming that a new component has those tolerances set accurately when the individual parts are pressed together. The clearance was literally, the very last thing I checked after:

scouring the water galleries, clearing the water drain at the back of the engine, rodding the radiator core, double checking the timing and ensuring the thermostat was working correctly ….

Of course, none of that other work hurt, but it did nothing to address the real problem which took me a total of 3 minutes to fix at the hydraulic press !  Much less time than writing and mucking about with a reseller who (at least over here, unless you are really lucky) will usually waste your time trying to discover difficulty with your complaint, rather than actually deal with the issue efficiently". -- David